Prefabricated components and chips lie at the heart of most analog and digital circuits. As these circuits become more prevalent and more complex, it has become increasingly important to those who manufacture and sell the component parts, as well as to those who purchase components and implement circuits using them, that these often delicate or sensitive components can be inspected efficiently and effectively, prior to installation. Similar demands exist with respect to other electrical and mechanical components.
Component suppliers traditionally ship their parts to the end user in various forms of transport packaging, the most popular being waffle trays or tape-and-reel arrangements.
In tape-and-reel arrangements, the tape is thermoformed of a thin layer of thermoplastic with a series of depressions or pockets formed along its length. A component part is inserted into a pocket and covered with a cover tape, which secures the part inside. The cover tape is usually a film or web with a thermally-activated or pressure-sensitive adhesive deposited on its underside. A length of the carrier/cover tape combination is then spooled onto a circular shipping reel. This system provides an efficient arrangement in which the components can be packaged, shipped and presented to an automated assembly process.
Waffle trays are similar, except the pockets are provided in a grid pattern on a thermoformed or injection molded tray. Instead of rolling the carrier onto a reel as with the tape, the trays are stacked for shipping and storage.
Circuit components are generally formed of a main body with a lead or one or more parallel series of leads extending from near the edges of the main body. In a typical component, the main body will be a regular parallelepiped (box-shaped), with sets of parallel leads extending out from one, two or four sides. When viewed from above, the longitudinal axis of each lead will be generally perpendicular to the side of the body from which it extends. The main body is generally more robust than the leads, and able to withstand forces typically encountered in formation, shipping and storage. The leads, however, are often quite delicate, and are more likely to be malformed or damaged during formation, shipping and storage.
Automated visual inspection ("AVI") systems have been developed for inspecting the leads while the component is within the carrier. The optics of a typical AVI system include an optical lens rested concentrically within a ring-shaped light source. The AVI system directs light from the ring light onto the carrier and component, either prior to affixation of the carrier cover, through the cover, or after removal of the cover. The AVI system electronically records an image through the lens of the entire carrier pocket and component therein, producing a gray-scale digital "snapshot". The AVI system then utilizes an algorithm to analyze the pixels in an inspection region in the digital snapshot (a two-dimensional region representative of a three-dimensional space surrounding the expected location of the ends of the leads and projected approximately perpendicular to the plane in which carrier cover normally lies) in order to identify light reflected back by the component. The typical AVI system interprets the location and brightness of the reflected light within the inspection region to determine whether the reflection pattern is consistent with leads that are properly formed.
An AVI system is generally designed to detect and interpret visible light (or other radiant energy) reflected directly by the component leads back toward the overhead lens. Such reflections may be referred to herein as "direct vertical reflections" or "direct return reflections". If the component is properly positioned in the inspection region, and the leads are properly constructed and have not been bent or broken, then the vertical reflections directly off the leads should all fall in a certain pattern a set distance from the component body. A typical lead, being metal, will create a brighter (or different wavelength) reflection than does the plastic carrier. Ordinarily, an AVI system will take an initial reading outside of the inspection region to determine where on the gray scale non-lead reflections will register. The system will then take a series of readings at discrete pixels or groups of pixels, located a predetermined distance apart, as it pans across the inspection region of the digital representation in a direction representative of the longitudinal axes of the leads being inspected, from the edge of the pocket toward the body of the component. Generally, the minimum longitudinal distance represented by each successive reading is equal to a pixel width, which is usually representative of about 0.05 millimeter. The system will typically be programmed to interpret the reception of a predetermined number of successive bright readings as indication of the presence of a lead.
However, an AVI system may also detect light reflected sideways by a non-horizontal surface of a component lead, such as a corner of the front edge of the lead, that rebounds vertically off the carrier. It may also detect light reflected directly upward by the carrier itself, at a corner. Either of these extraneous reflections can cause false or misleading readings. Thus, in order to improve the reliability of AVI systems, it is important to reduce the instances of extraneous reflections in the inspection region, or at least insure that any extraneous reflections are sufficiently removed from one another and the actual lead ends to avoid creating a false series of successive bright readings.
For example, FIG. 4A schematically shows a component C in a standard carrier pocket with rounded comers and near-vertical end walls. If an AVI system were inspecting the leads on the right side of the pocket (in this view), it would begin analyzing a digital representation of the pocket at a location depicting the right end of the pocket and scan toward the left, as if it were panning across the actual pocket in the direction indicated by the arrow in the figure. In this case, the first fairly bright reflection encountered would be a vertical reflection at the pocket edge (P1), which would be a false reading, as far as the presence of a lead is concerned. Next would be a vertical reflection at the comer between the pocket wall and the pocket floor (P2), which also would be a false reading. Next would be the first true reading, the vertical reflection near the end of the lead (P3). Because the pockets end wall is substantially vertical and spaced from the leads, for the sake of this illustration, it will be assumed that no sideways reflection off the lead rebounds from the end wall up to the AVI. While the reflection at P1 is separated by some distance from the reflection at the end of the lead (P3), the reflection at P2 is quite close to the reflection at P1, which could be incorrectly interpreted by an AVI system to indicate the beginning of the lead, which, were it true, might indicate that the lead is too long or is not properly bent. The result might be rejection of a component that is not defective at all.
As another example, FIG. 4B schematically shows a component C in another standard carrier pocket, this pocket having a more arcuate shape, with sloped end walls. If an AVI system were inspecting the leads on the right side of this pocket (in this view), it would begin analyzing a digital representation of the pocket at a location depicting the right end of the pocket and scan toward the left, as if panning across the actual pocket in the direction indicated by the arrow in the figure. As with the previous case, the first fairly bright vertical reflection encountered would be at the pocket edge (P1). However, the next reflections would be a pair of sideways reflections off the lead, that rebound vertically off the sloped end wall, back to the AVI system (P4, P5). In this case, the reflection at P4 is off the "elbow" of the lead, while the reflection at P5 is off the top edge along the front end of the lead. Next is the reflection directly off a horizontal portion of the lead, adjacent its front end (P3), then the reflection off the carrier at the location where the pocket "bottoms out" (P6). While the reflection at P1 is separated by some distance from the direct return reflection caused by the horizontal surface adjacent the end of the lead (P3), the "doublet" reflections at P4 and P5 are generally close enough to one another, and to the reflection at P3, to be incorrectly interpreted by an AVI system as indicating the beginning of the lead. An actual lead beginning at the point of the P4 reflection would either be too long or not properly bent. Therefore, the AVI system might again reject a component that is not defective.
Conversely, it is possible that a truly misshapen lead would create doublet reflections that rebound vertically from a point lower on the pocket wall, with the result that they combine with the reflection at P6 to falsely give the impression of a properly formed lead. This could lead to the acceptance of a malformed lead. In either case, if the entire component were to shift slightly to the right (in this view) within the pocket, so that the lead contacted the inside of the pocket, the reflections at P3, P4 and P5 would be crowded even closer together, compounding the problem.
FIG. 4C schematically shows a component C in a standard carrier pocket with slightly sloped sides (only a few degrees from vertical). If an AVI system were inspecting the leads on the right side of this pocket (in this view), it would begin analyzing a digital representation of the pocket at a location depicting the right end of the pocket and scan toward the left, as if panning across the actual pocket in the direction indicated by the arrow in the figure. As with the earlier examples, the directly vertical reflection at the pocket edge (P1) would be first encountered. The next would be the sideways reflection off the end of the lead that rebounds off the end wall to the AVI system (P5). Ignoring a probable reflection at the location at which the pocket "bottoms out," the next is the vertical reflection directly off the horizontal segment just inboard of the end of the lead (P3). The reflections at P1, P5 and P3 are generally close enough to one another to be incorrectly interpreted by an AVI system as indicating an out-of-spec elongation of the lead. Therefore, the AVI system might again reject a component that is not defective. As with the previous example, discussed in connection with FIG. 4B, if the component were to shift to the right, the reflections at P3 and P5 would be moved even closer to one another.
Because an AVI system will also usually need to detect and interpret an ink or etched symbol and/or an off-center notch (to establish the type and orientation of the component) on the component body in order to properly interpret the reflection pattern in the inspection zone, the extraneous reflection problem cannot be acceptably addressed simply by reducing the intensity of the light.
Further, due to dimension tolerances in the manufacture of carrier tape or trays, as well as in the indexing machines that transport the tape or trays through the inspection systems, a certain amount of drift in the actual versus expected location of the leads will sometimes occur. Thus, it is preferred that the inspection region be as large as possible to account for such drift.
Examples of known carrier tape designs are shown in U.S. Pat. No. 4,966,281, to Kawanishi et al., which relates to an electronic component carrier tape with a number of cavities or pockets. FIGS. 4A-11C of this patent illustrate several carrier pocket configurations. For example, FIGS. 4A, 4B and 8A-11C show various rectilinear pockets, while FIGS. 5 and 6A-7B show curved pocket structures. However, none of these designs sufficiently addresses the above-discussed reflection problems. For example, it is apparent from inspection that the pocket shown in FIG. 5 of Kawanishi et al. has the edge of the pocket floor near the lead tips. Thus, this design likely suffers from at least the bottom-edge reflection problem discussed above in correction with FIG. 4A herein, as will be discussed more fully below.
Further examples of known carrier tape designs are shown in Japanese Utility Model Disclosure No. 4-80890, by Fugiwara. Each of the figures in this utility model shows a cross-section of an arcuate pocket structure. By inspection, it is apparent that each of the designs shown is sloped severely enough in the vicinity of the lead tips that it likely suffers from at least the "doublet" reflection problem discussed above in connection with FIG. 4B herein, as will be discussed more fully below.
Thus, there is a need in the art for a component carrier pocket designed to reduce or eliminate extraneous reflections in the inspection region of an automated visual inspection system.
There is a further need for a component carrier pocket designed to separate component lead direct vertical reflections from the rebound vertical reflections off the pocket walls.
There is also a need for a component carrier pocket designed to reduce or eliminate detectable (in most cases, substantially vertical) reflections within the inspection region directly off the pocket walls.
There is a still further need for a component carrier pocket designed to place as much distance as is reasonably possible between the extraneous reflections and the direct vertical reflections off the actual lead ends.